Laser weld method and weld structure

ABSTRACT

A laser welding method includes: forming a melted section in which plural metallic workpieces within a weld region are melted by projecting a first laser beam onto said weld region with parts of the workpieces being set as the weld region when the workpieces are welded; and projecting a second laser beam while the melted section is being solidified or after a solidified section in which the melted section is solidified is formed such that the second laser beam circles around center of the melted section or the solidified section from an irradiation start position deviated from said center toward said center or such that the second laser beam is focused to the center of the melted section or the solidified section from an irradiation start region that includes the center and a periphery thereof.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-079702 filed onApr. 8, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a laser welding method that is favorable forwelding plural workpieces with use of a laser beam and a weld structure.

2. Description of Related Art

For example, workpieces of two metal plates are stacked or abutted andirradiated with a laser beam for laser welding. In order to increasereliability and intensity by the laser welding, for example, in JapanesePatent Publication No. 3-80596, a technique of a laser welding methodfor projecting a laser beam onto a weld region with parts of surfaces ofplural metallic workpieces being set as the weld region when theworkpieces are welded is suggested.

In this technique, the laser beam is projected (scanned) to circlearound center of the weld region. In this way, the workpieces can bemelted together by being applied with a uniform amount of heat to a widerange. As a result, reliability of a weld section welded by the laserbeam can be increased.

However, in the case where parts of the workpieces are melted byprojecting the laser beam as described in Japanese Patent PublicationNo. 3-80596, heat is more likely to be dissipated from a periphery of amelted section (a molten pool) that has been melted rather than thecenter of the melted section, and thus solidification of the meltedsection starts from the periphery of the melted section and progressestoward the center of the melted section. At this time, a solidificationrate from start of solidification of the center of the melted section tocompletion of solidification (that is, a cooling rate) is higher than asolidification rate (a cooling rate) of the periphery of the meltedsection.

As a result, there is a case where solidification shrinkage of thecenter of the weld section is completed before being compensated byliquid-phase flow and where this causes generation and extension of acrack that starts from the center of the weld section (a solidifiedsection), in which the melted section is solidified, or the vicinitythereof. This crack extends from the center of the weld section (thesolidified section), and thus a breaking mode thereof is difficult to bepredicted. Especially when the workpieces made of aluminum alloy areused, such a phenomenon is significant.

SUMMARY OF THE INVENTION

The invention provides a laser welding method and a weld structure thatcan reduce a crack in a solidified section in which a melted section issolidified.

A first aspect of the invention relates to a laser welding method. Thelaser welding method includes: forming a melted section in which pluralmetallic workpieces within a weld region are melted by projecting afirst laser beam onto said weld region with parts of surfaces of theworkpieces being set as the weld region when the workpieces are welded;and projecting a second laser beam while the melted section is beingsolidified or after a solidified section in which the melted section issolidified is formed such that the second laser beam circles aroundcenter of the melted section or the solidified section from anirradiation start position deviated from said center toward said centeror such that the second laser beam is focused to the center of themelted section or the solidified section from an irradiation startregion that includes the center and a periphery thereof.

According to the above aspect, in irradiation of the first laser beam,the first laser beam is projected onto the weld region, so as to formthe melted section (a molten pool) in which the workpieces in the weldregion are melted. In irradiation of the second laser beam, in the casewhere the second laser beam is projected until the melted section issolidified, when the second laser beam circles around the center of themelted section from the irradiation start position that is deviated fromthe center toward the center, solidification of the center of the meltedsection can be delayed. Similarly, also when the second laser beam isfocused to the center from the irradiation start region that includesthe center of the melted section and the periphery thereof, thesolidification of the center of the melted section can be delayed.Accordingly, the liquid flows along with solidification shrinkage of thecenter of the melted section, and thus generation of a crack in thevicinity of the center of the melted section can be suppressed.

Meanwhile, in the irradiation of the second laser beam, in the casewhere the second laser beam circles around the center from theirradiation start position that is deviated from the center of thesolidified section toward the center after the solidified section inwhich the melted section is solidified is formed, an irradiated part ismelted again, and thus the crack that is formed from the center of thesolidified section toward a peripheral edge of the solidified sectioncan be sealed. Similarly, also when the second laser beam is focused tothe center from the irradiation start region that includes the center ofthe solidified section and the periphery thereof, the crack that isformed from the center of the solidified section toward the peripheraledge of the solidified section can be sealed.

Here, the “center of the melted section” in the invention refers to apart of the melted section that is solidified at the end whilesolidification of the melted section is started from a peripherythereof. The “center of the solidified section” refers to a part of themelted section before the solidification that is solidified at the endwhile the solidification thereof is started from the periphery thereof.

When the second laser beam is projected until the melted section issolidified, the second laser beam is projected onto a melted part of themelted section. Meanwhile, when the second laser beam is projected afterthe solidified section in which the melted section is solidified isformed, the second laser beam is projected onto a solidified section. Itis because the melted section is completely solidified.

Here, in the above aspect, the second laser beam may be projected untilthe melted section is solidified. In the irradiation of the second laserbeam, the second laser beam is projected onto the melted section alongwith the progression of the solidification in a direction from theperipheral edge of the melted section toward the center of the meltedsection. In this way, the progression of the solidification in thedirection from the peripheral edge of the melted section toward thecenter of the melted section may be delayed.

According to the above aspect, with respect to progressing thesolidification of the melted section from the peripheral edge thereoftoward the center after the irradiation of the first laser beam, themelted section is irradiated with the second laser beam. Accordingly,the melted section is heated by the second laser beam, and theprogression of the solidification in the direction from the peripheraledge of the melted section toward the center of the melted section canbe delayed. In this way, liquid flows along with solidificationshrinkage of the melted section, and thus generation of the crack in thevicinity of the center of the melted section can be suppressed.

Here, in the above aspect, in the irradiation of the second laser beam,the irradiation start position may be located on the peripheral edge ofthe melted section. In this case, in addition to the above-describedeffect, a fluctuation in a solidification rate (a cooling rate) can besuppressed from the peripheral edge of the melted section to the centerof the melted section, and thus the melted section can be solidified tohave a further uniform structure. Similarly, the irradiation startregion may be a region that is surrounded by the peripheral edge of themelted section. Also, in this case, the melted section can be solidifiedto have a further uniform structure from the peripheral edge of themelted section to the center of the melted section.

Furthermore, in the above aspect, in the irradiation of the second laserbeam, the irradiation start position may be located between theperipheral edge of the melted section and the center of the meltedsection. In this case, the progression of the solidification of at leastthe center of the melted section and the periphery thereof can bedelayed. Accordingly, in addition to the above-described effect, thesolidification rate (the cooling rate) at the center and the peripherythereof can be brought closer to the solidification rate (the coolingrate) in the vicinity of the peripheral edge of the melted section.Similarly, the irradiation start region may be a region, the peripheraledge of which is located between the peripheral edge of the meltedsection and the center of the melted section and which includes thecenter. The melted section can be solidified so as to have the uniformstructure from the peripheral edge of the melted section to the centerof the melted section.

In the above aspect, in the irradiation of the second laser beam, thesecond laser beam may be projected such that the columnar crystalstructure grows from the peripheral edge of the melted section to thecenter of the melted section in conjunction with the progression of thesolidification. In the case of any aspect, the solidification rate atany parts that are located from the peripheral edge of the meltedsection to the center of the melted section can be brought closer toeach other. Thus, the solidified section in which the melted section issolidified (the weld section) is formed of the columnar crystalstructure from the peripheral edge of the solidified section to thecenter of the solidified section, and in the columnar crystal structure,the columnar crystal extends in a direction from the peripheral edge ofthe solidified section toward the center of the solidified section. As aresult, in the columnar crystal structure of the solidified section,even when the crack that starts from the center is generated, extensionof the crack can be reduced. It is because growth of each of thecolumnar crystals from the peripheral edge of the solidified section tothe center of the solidified section is intermittent.

In the above aspect, in a period after the irradiation of the firstlaser beam and before the irradiation of the second laser beam, a secondlaser beam irradiation may be started after the columnar crystalstructure grows from the peripheral edge of the melted section towardthe center of the melted section so as to surround the center of themelted section in conjunction with the progression of the solidificationand then the growth of the equiaxed crystal structure is started afterthe growth of said columnar crystal structure is completed. In saidsecond laser beam irradiation process, the second laser beam may beprojected such that the equiaxed crystal structure remains in a mannerto surround the center of the melted section and that the columnarcrystal structure grows from said equiaxed crystal structure to thecenter of the melted section.

According to the above aspect, the solidified section in which themelted section is solidified (the weld section) includes: a firstcolumnar crystal region that is formed of the columnar crystal structurein which the columnar crystal extends from the peripheral edge of thesolidified section in the direction from the peripheral edge of thesolidified section toward the center of the solidified section; anequiaxed crystal region that is formed of the equiaxed crystal structureformed to surround the center of the solidified section from the firstcolumnar crystal region; and a second columnar crystal region that isformed of the columnar crystal structure from the equiaxed crystalregion to the center of the solidified section, and in the columnarcrystal structure, the columnar crystal extends toward the center of thesolidified section. In this way, even when the crack that starts fromthe center is generated in the columnar crystal structure of the secondcolumnar crystal region during the solidification, the extension of thecrack can be reduced. It is because the growth of the columnar crystalis intermittent. In addition, even when the crack further extends, thiscrack can be stopped by the equiaxed crystal structure of the equiaxedcrystal region. As a result, the extension of the crack can besuppressed.

In addition, in the above-described aspect, the case where the secondlaser beam is projected until the melted section is solidified isdescribed. Meanwhile, an aspect of the case where the second laser beamis projected after the solidified section in which the melted section issolidified is formed will be described below.

In the above aspect, in the irradiation of the second laser beam, thesecond laser beam is projected onto the solidified section in which themelted section is solidified so as to melt the solidified section again.In this case, a position on a peripheral edge of a region that includesthe center of the solidified section and the vicinity thereof may be setas the irradiation start position such that the cooling rate, in whichthe region that includes the center of the solidified section and thevicinity thereof and that is melted again by the irradiation of thesecond laser beam is solidified, is slower than the cooling rate, inwhich the region that includes the center of the melted section and thevicinity thereof is solidified after the irradiation of the first laserbeam. According to this aspect, the crack is likely to be generated atthe center of the solidified section and the vicinity thereof. However,a region from the irradiation start position to the center of thesolidified section is melted again, and thus the crack that is formed atthe center of the solidified section and the vicinity thereof can besealed.

A region that is surrounded by the center of the solidified section andthe peripheral edge of the region in the vicinity thereof may be set asthe irradiation start region, and the second laser beam may be projectedthereon. In this case, the same effect can be expected because anirradiated part in the irradiation start region is melted again, andthus the crack that is formed from the center of the solidified sectiontoward the peripheral edge of the solidified section can be sealed.

In the above aspect, the solidified section may include: the equiaxedcrystal region that is formed with an equiaxed crystal structure in amanner to include the center of the solidified section; and the columnarcrystal region that is formed with a columnar crystal structure in amanner to surround the equiaxed crystal region from the peripheral edgeof the solidified section toward the equiaxed crystal region. The secondlaser beam may be projected such that the equiaxed crystal structure ofthe equiaxed crystal region becomes the columnar crystal structure.

According to the above aspect, with the irradiation of the second laserbeam, the entire equiaxed crystal structure becomes the columnar crystalstructure in a manner to include the center of the solidified section.Thus, the solidified section is formed of the columnar crystal structurefrom the peripheral edge of the solidified section to the center of thesolidified section, and in the columnar crystal structure, the columnarcrystal extends in the direction from the peripheral edge of thesolidified section toward the center of the solidified section. As aresult, in the columnar crystal structure of the solidified section,even when the crack that starts from the center is generated, theextension of the crack can be reduced. It is because the growth of eachof the columnar crystals from the peripheral edge of the solidifiedsection to the center of the solidified section is intermittent.

In addition, in the above aspect, the solidified section may include:the equiaxed crystal region that is formed with an equiaxed crystalstructure in a manner to include the center of the solidified section;and the columnar crystal region that is formed with a columnar crystalstructure in a manner to surround the equiaxed crystal region from theperipheral edge of the solidified section toward the equiaxed crystalregion. The second laser beam may be projected onto the equiaxed crystalregion such that a part of the equiaxed crystal structure of theequiaxed crystal region surrounds the center of the solidified sectionand that the remaining equiaxed crystal structure of the equiaxedcrystal region becomes the columnar crystal structure.

According to the above aspect, the solidified section (the weld section)includes: the first columnar crystal region that is formed of thecolumnar crystal structure in which the columnar crystal extends fromthe peripheral edge of the solidified section in the direction from theperipheral edge of the solidified section toward the center of thesolidified section; the equiaxed crystal region that is formed of theequiaxed crystal structure formed to surround the center of thesolidified section from the first columnar crystal region; and thesecond columnar crystal region that is formed of the columnar crystalstructure from the equiaxed crystal region to the center of thesolidified section, and in the columnar crystal structure, the columnarcrystal extends toward the center of the solidified section. In thisway, even when the crack that starts from the center is generated in thecolumnar crystal structure of the second columnar crystal region duringthe solidification, the extension of the crack can be reduced. It isbecause the growth of the columnar crystal is intermittent. In addition,even when the crack further extends, this crack can be stopped by theequiaxed crystal structure of the equiaxed crystal region. As a result,the extension of the crack can be suppressed.

A second aspect of the invention relates to a weld structure. The weldstructure includes a weld section in which parts of plural metallicworkpieces are melted by a laser beam and welded. The weld section isformed of a columnar crystal structure from a peripheral edge of theweld section to center of the weld section, and in the columnar crystalstructure, a columnar crystal extends in a direction from the peripheraledge of the weld section toward the center of the weld section.

According to the above aspect, the weld section is formed of thecolumnar crystal structure from the peripheral edge of the weld sectionto the center of the weld section, and in the columnar crystalstructure, the columnar crystal extends in the direction from theperipheral edge of the weld section toward the center of the weldsection. Thus, in the columnar crystal structure of the weld section,even when the crack that starts from the center is generated, theextension of the crack can be reduced. It is because growth of each ofthe columnar crystals from the peripheral edge of the weld section tocenter of the weld section is intermittent.

In addition, a third aspect of the invention relates to a weldstructure. The weld structure includes a weld section in which parts ofplural metallic workpieces are melted by a laser beam and welded. Theweld section includes: a first columnar crystal region that is formed ofa columnar crystal structure in which a columnar crystal extends from aperipheral edge of the weld section in a direction from the peripheraledge of the weld section toward a center of the weld section; anequiaxed crystal region that is formed of an equiaxed crystal structureformed to surround the center of the weld section from the firstcolumnar crystal region; and a second columnar crystal region that isformed of the columnar crystal structure from said equiaxed crystalregion to the center of the weld section, and in the columnar crystalstructure, the columnar crystal extends toward the center of the weldsection.

According to the above aspect, even when a crack that starts from thecolumnar crystal structure of the second columnar crystal region inwhich the crack is likely to be generated during weld is generated,extension of the crack can be reduced. It is because the growth of thecolumnar crystal is intermittent. In addition, even when the crackfurther extends, this crack can be stopped by the equiaxed crystalstructure of the equiaxed crystal region. As a result, the extension ofthe crack can be suppressed.

Furthermore, a fourth aspect of the invention relates to a weldstructure. The weld structure includes a weld section in which parts ofplural metallic workpieces are melted by a laser beam and welded. Asurface of the weld section on a side that is irradiated with the laserbeam is formed of: a primary recessed surface that is recessed from aperipheral edge of the weld section toward a center of the weld section;and a secondary recessed surface that is further recessed from saidprimary recessed surface in the vicinity of the center of said weldsection. A back surface of the weld section that corresponds to thesecondary recessed surface is not recessed or is a shallower recessedsurface than the secondary recessed surface.

In the weld structures according to the above aspects, such a surfaceshape may be formed in the case where the weld structure is welded bythe weld method of the laser beam as described above. In a general weldmethod, a large recess is formed in the back surface on the oppositeside from the surface irradiated with the laser beam. However, in theabove aspects of the invention, the surface on the side that isirradiated with the laser beam is formed with: the primary recessedsurface that is recessed from the peripheral edge of the weld sectiontoward the center of the weld section; and the secondary recessedsurface that is further recessed from the primary recessed surface inthe vicinity of the center of the weld section. Accordingly, thesecondary recessed surface can easily be repaired by a sealer or thelike after the weld. In addition, the back surface of the weld sectionthat corresponds to the secondary recessed surface is not recessed or isthe shallower recessed surface than the secondary recessed surface.Thus, water or the like is less likely to be collected, and a chance ofcorrosion of the part can be reduced.

According to the above aspect of the invention, a crack that isgenerated in the weld section located in the parts of the workpieces,that is, in the solidified section in which the melted section issolidified can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1A is a schematic view of one example of a laser welding device forimplementing a laser welding method according to a first embodiment ofthe invention and is a view of a weld state in a lateral direction;

FIG. 1B is a schematic view of the one example of the laser weldingdevice for implementing the laser welding method according to the firstembodiment of the invention and is a view of the weld state in a frontdirection;

FIG. 2 includes schematic views for illustrating a first laser beamirradiation process according to the laser welding method of the firstembodiment of the invention, in which

FIG. 2A to FIG. 2C show that workpieces are irradiated with a firstlaser beam in this order;

FIG. 3 includes schematic views of a state in which a melted section iscooled without performing a second laser beam irradiation process afterthe irradiation process shown in FIG. 2, in which FIG. 3A and FIG. 3Bshow that the melted section is cooled in this order;

FIG. 4 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of the firstembodiment of the invention, in which FIG. 4A to FIG. 4D show that theworkpieces are irradiated with a second laser beam in this order and inwhich FIG. 4E is a view of the weld section after the irradiation of thesecond laser beam;

FIG. 5 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the first embodiment of the invention, in which FIG. 5Aillustrates the irradiation of the second laser beam, in which FIG. 5BBto FIG. 5BD show that the workpieces are irradiated with the secondlaser beam in this order, and in which FIG. 5BE is a view of the weldsection after the irradiation of the second laser beam;

FIG. 6 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of a secondembodiment of the invention, in which FIG. 6A and FIG. 6B are views of achange in solidification of the melted section, in which FIG. 6C andFIG. 6D show that the workpieces are irradiated with the second laserbeam in this order, and in which FIG. 6E is a view of the weld sectionafter the irradiation of the second laser beam;

FIG. 7 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the second embodiment of the invention, in which FIG. 7Aand FIG. 7B are views of the change in the solidification of the meltedsection, in which FIG. 7C and FIG. 7D show that the workpieces areirradiated with the second laser beam in this order, and in which FIG.7E is a view of the weld section after the irradiation of the secondlaser beam;

FIG. 8 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of a thirdembodiment of the invention, in which FIG. 8A is a view of thesolidified section after the irradiation of the first laser beam, inwhich FIG. 8B and FIG. 8C show that the workpieces are irradiated withthe second laser beam in this order, and in which FIG. 8D is a view ofthe solidified section after the irradiation of the second laser beam;

FIG. 9 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the third embodiment of the invention, in which FIG. 9Ais a view of the solidified section after the irradiation of the firstlaser beam, in which FIG. 9B and FIG. 9C show that the workpieces areirradiated with the second laser beam in this order, and in which FIG.9D is a view of the solidified section after the irradiation of thesecond laser beam;

FIG. 10A is an image of a structure of a weld section according to afirst example of the invention;

FIG. 10B is an image of the structure of the weld section according to asecond example of the invention;

FIG. 10C is an image of the structure of the weld section according to acomparative example;

FIG. 11A is an image of a cross section of the weld section according tothe first example of the invention;

FIG. 11B is an image of the cross section of the weld section accordingto the comparative example;

FIG. 12A is a graph of maximum lengths of fractures in the weld sectionsaccording to the first example, the second example, and the comparativeexample of the invention;

FIG. 12B is a graph of total lengths of the fractures in the weldsections according to the first example, the second example, and thecomparative example of the invention;

FIG. 13A is an image of a surface of the fracture in the weld sectionaccording to the second example of the invention when a shearing loadacts on the weld section;

FIG. 13B is an image of the surface of the fracture in the weld sectionaccording to the comparative example when the shearing load acts on theweld section;

FIG. 14A is an image of the surface of the fracture in the weld sectionaccording to the second example of the invention when a tensile loadacts on the weld section; and

FIG. 14B is an image of the surface of the fracture in the weld sectionaccording to the comparative example when the tensile load acts on theweld section.

DETAILED DESCRIPTION OF EMBODIMENTS

A laser welding method according to some embodiments of the inventionwill be described below.

First Embodiment

FIG. 1 is a schematic view of one example of a laser welding device forimplementing a laser welding method according to a first embodiment ofthe invention. FIG. 1A is a view of a weld state in a lateral direction,and FIG. 1B is a view of the weld state in a front direction.

1. Regarding a Device Configuration

FIG. 1 is a schematic view of an overall configuration of a laserwelding device 100 according to the embodiment of the invention. FIG. 1Ais a view of the weld state in the lateral direction, and FIG. 1B is aview of the weld state in the front direction.

The laser welding device shown in FIG. 1A and FIG. 1B includes a laserbeam irradiation section 1 as a main component. The laser beamirradiation section 1 is a device that provides laser beams for welding(first and second laser beams) L1, L2 and projects a selected laser beamonto two metallic workpieces W1, W2 that are stacked or arranged with aslight gap interposed therebetween. In this embodiment, the twoworkpieces W1, W2 are welded by being stacked. However, the number ofthe workpieces is not limited to two. For example, the two workpiecesmay be subject to butt welding or fillet welding in a method, which willbe described below.

In addition, as a material of the workpieces W1, W2, a material that caneasily be cracked, such as aluminum alloy or steel (high-carbon steel,for example), is preferred. A columnar crystal structure and an equiaxedcrystal structure are likely to be formed in any of these materials bywelding, which will be described below, and a crack is easily generatedat the center of a weld section. However, such a problem can be solvedby performing laser welding, which will be described below.

The first and second laser beams L1, L2, which will be described below,are each sequentially reflected by a fixed mirror 7 and a driven mirror8 as optical systems and projected with respect to the two workpiecesW1, W2. Here, the driven mirror 8 is controlled to be driven so that adirection of reflection of the first laser beam L1 incident on thedriven mirror 8 is controlled and that the first and second laser beamsL1, L2 are projected onto a desired position. These beams can be scannedin a trajectory (for example, in a circular shape or a helical shape)that is set in advance as shown in FIG. 1B, for example. First andsecond laser beam irradiation processes, which will be described below,are performed by using such a laser welding device 100.

2. Regarding the First Laser Beam Irradiation Process

FIG. 2 includes schematic views for illustrating the first laser beamirradiation process according to the laser welding method of the firstembodiment, in which FIG. 2A to FIG. 2C show that the workpieces areirradiated with the first laser beam in this order.

As shown in FIG. 2, in the first laser beam irradiation process, whenthe two metallic workpieces W1, W2 are welded, parts of the workpiecesare set as a weld region P, and the weld region P is irradiated with thefirst laser beam L1. In this way, a melted section Y in which theworkpieces in the weld region P are melted is formed.

More specifically, in this embodiment, as shown in FIG. 2A, the firstlaser beam L1 is scanned on a periphery R1 such that the first laserbeam L1 circles around center of the weld region P, and melts theworkpieces in the periphery R1 of the weld region P.

Next, as shown in FIG. 2B, the first laser beam L1 is scanned on aperiphery R2, a radius of which is larger than that of the periphery R1,such that the first laser beam L1 circles around the center of the weldregion P, and melts the workpieces in the vicinity of the periphery R2of the weld region P.

Furthermore, as shown in FIG. 2C, the first laser beam L1 is scanned ona periphery R3, a radius of which is larger than that of the peripheryR2, such that the first laser beam L1 circles around the center of theweld region P, and melts the workpieces in the vicinity of the peripheryR3 of the weld region P. Just as described, the first laser beam L1circles around the center from the center of the weld region P toward aperipheral edge of the weld region P, and the material in the region ismelted by the first laser beam L1.

In this way, in the melted section Y that is formed, a temperature atthe peripheral edge is higher than a temperature at the center. Thus,compared to a case where the first laser beam L1 is scanned from theperipheral edge to the center, a solidification rate at the center canbe lowered. Just as described, the center is gradually solidified in thesame manner as the peripheral edge. Thus, it is possible to suppress acrack from generating in the vicinity of the center after the weldsection is solidified.

Here, in the case where the second laser beam irradiation process, whichwill be described below, is not performed after the irradiation processshown in FIG. 2 and the melted section Y is cooled, as shown in FIG. 3A,heat is more easily radiated from a peripheral edge S of the meltedsection Y than from center C thereof. Accordingly, solidification startsfrom the peripheral edge S of the melted section Y, and thesolidification of the melted section Y progresses toward the center.

Then, as shown in FIG. 3B, in a solidified section G in which the meltedsection Y is solidified, an equiaxed crystal structure G2 (an equiaxedcrystal region) is formed in a manner to include the center C of thesolidified section G, and a columnar crystal structure (a columnarcrystal region) G1 is formed from the peripheral edge S of thesolidified section G toward the equiaxed crystal region in a manner tosurround the equiaxed crystal structure G2 (the equiaxed crystalregion).

Here, irrespective of formation of the equiaxed crystal structure andthe columnar crystal structure, a solidification rate (that is, acooling rate) from the start of the solidification of the center C ofthe melted section Y to the complete solidification is higher than asolidification rate (a cooling rate) of the peripheral edge S of themelted section Y. As a result, solidification shrinkage of the center Cof the melted section Y is completed before being compensated byliquid-phase flow at the center C, and the structure is pulled in acircumferential direction. This possibly causes generation and extensionof the crack that starts from the center of the solidified section G orthe vicinity thereof.

In view of the above, in this embodiment, the second laser beamirradiation process, which will be described below, is performed. Notedthat, in this embodiment, the melted section (a molten pool) Y is formedby scanning the first laser beam L1. However, a method for forming themelted section and the like are not particularly limited as long as themelted section Y with the peripheral edge as shown in FIG. 2C can beformed.

3. Regarding the Second Laser Beam Irradiation Process

FIG. 4 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of the firstembodiment, in which FIG. 4A to FIG. 4D show that the workpieces areirradiated with a second laser beam in this order and in which FIG. 4Eis a view of the weld section after the irradiation of the second laserbeam.

In this embodiment, in the second laser beam irradiation process, thesecond laser beam L2 is projected onto a melted part in the meltedsection Y until the melted section Y is solidified. More specifically,the second laser beam L2 circles around the center C from an irradiationstart position that is deviated from the center C of the melted sectionY toward the center C, and then the second laser beam L2 is focused tothe center C.

More specifically, as shown in FIG. 4A, in this embodiment, theirradiation start position is located on the peripheral edge S of themelted section Y. The second laser beam L2 is projected onto the meltedsection Y from this position while circling around the center C suchthat the columnar crystal structure grows from the peripheral edge S ofthe melted section Y to the center of the melted section Y along withprogression of the solidification in a direction from the peripheraledge S of the melted section Y toward the center C of the melted sectionY. In this case, the second laser beam L2 may be projected to drawconcentric circles with gradually reduced radii with the center C beingthe center. Alternatively, the second laser beam L2 may be projected ina helical shape toward the center C.

As described with reference to FIG. 3A, after the irradiation of thefirst laser beam, the solidification of the melted section Y isprogressed from the peripheral edge S toward the center C. However, asdescribed above, the progression of the solidification in the directionfrom the peripheral edge S of the melted section Y toward the center Cof the melted section Y is delayed by projecting the second laser beamL2 onto the melted section Y.

In this way, a fluctuation in the solidification rate (the cooling rate)from the peripheral edge S of the melted section Y to the center C ofthe melted section Y is suppressed so that the melted section Y can besolidified to have the further uniform structure. In this embodiment,the solidified section G in which the melted section Y is solidified isformed of the columnar crystal structure from the peripheral edge S ofthe solidified section G to the center C of the solidified section G,and in the columnar crystal structure, a columnar crystal extends in adirection from the peripheral edge S of the solidified section G towardthe center C of the solidified section G (for example, see FIG. 10A,which will be described below).

As a result, in the columnar crystal structure of the solidified sectionG, even when the crack that starts from the center C is generated, theextension of the crack can be reduced. It is because the growth of eachof the columnar crystals from the peripheral edge S of the solidifiedsection G to the center of the solidified section G is intermittent.

FIG. 5 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the first embodiment, in which FIG. 5A illustrates theirradiation of the second laser beam, in which FIG. 5BB to FIG. 5BD showthat the workpieces are irradiated with the second laser beam in thisorder, and in which FIG. 5BE is a view of the weld section after theirradiation of the second laser beam.

In the above-described embodiment, the second laser beam L2 is scannedon the melted section Y. For example, as shown in FIG. 5A, a position ofa focal point of the second laser beam L2 may be adjusted so as toadjust the irradiation region of the second laser beam L2 with respectto the workpieces. In this case, the irradiation region (an irradiationarea) can be adjusted by moving a condenser lens of the second laserbeam L2 at a high speed. Here, the second laser beam L2 is projectedfrom a position T2 (T2′) as a defocused position to a focused positionof T1 while output of the laser beam is adjusted such that intensitythereof becomes constant, for example.

In this embodiment, the second laser beam L2 is projected such that thesecond laser beam L2 is focused from the irradiation start region, whichincludes the center C of the melted section Y and the periphery thereof,to the center C. More specifically, as shown in FIG. 5BB, theirradiation start region is set as a region that is surrounded by theperipheral edge S of the melted section Y. Then, as shown in FIG. 5BB to5BD, the progression of the solidification in the direction from theperipheral edge S of the melted section Y toward the center C of themelted section Y is delayed such that the columnar crystal structuregrows from the peripheral edge S of the melted section Y to the center Cof the melted section Y by focusing the second laser beam L2 to themelted section Y along with the progression of the solidification in thedirection from the peripheral edge S of the melted section Y toward thecenter C of the melted section Y. As a result, the same effect as thatobtained in the above-described case can be expected.

In this embodiment and the modification thereof, the second laser beamL2 is projected such that the columnar crystal structure grows from theperipheral edge S of the melted section Y to the center C of the meltedsection Y. However, for example, in the case where the progression ofthe solidification in the direction from the peripheral edge S of themelted section Y toward the center C of the melted section Y is delayedand where the liquid can flow along with the solidification shrinkage ofthe center C of the melted section Y, the extension of the crack at thecenter C can be suppressed. Thus, the irradiation of the second laserbeam L2 along with the growth of the structure during the solidificationis not necessarily required.

Second Embodiment

FIG. 6 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of a secondembodiment, in which FIG. 6A and FIG. 6B are views of a change in thesolidification of the melted section, in which FIG. 6C and FIG. 6D showthat the workpieces are irradiated with the second laser beam in thisorder, and in which FIG. 6E is a view of the weld section after theirradiation of the second laser beam.

The second embodiment differs from the first embodiment in theirradiation start position of the second laser beam L2 and timing of thesecond laser beam L2. Noted that the second embodiment is the same asthe first embodiment in a point that, in the second laser beamirradiation process, the melted part in the melted section Y isirradiated with the second laser beam L2 until the melted section Y issolidified. The same points of the second embodiment as the firstembodiment will not be described in detail.

In the second embodiment, in the second laser beam irradiation process,the irradiation start position is located between the peripheral edge Sof the melted section Y and the center C of the melted section Y (seeFIG. 6C, for example). Here, in this embodiment, in a period after thefirst laser beam irradiation process (see FIG. 2C) and before theirradiation of the second laser beam (see FIG. 6C), as shown in FIG. 6Aand FIG. 6B, the columnar crystal structure G1 grows from the peripheraledge S of the melted section Y toward the center C of the melted sectionY in a manner to surround the center C of the melted section Y inconjunction with the progression of the solidification. At this time,the second laser beam is not projected.

Next, after the growth of the equiaxed crystal structure G2 is startedafter the growth of the columnar crystal structure G1 is completed, asshown in FIG. 6C, the second laser beam irradiation process is started.In the second laser beam irradiation process, the second laser beam L2is projected such that the equiaxed crystal structure G2 remains tosurround the center C of the melted section Y and that the columnarcrystal structure grows from the equiaxed crystal structure G2 to thecenter C of the melted section Y. In this case, the second laser beam L2may be projected to draw concentric circles with gradually reduced radiiwith the center C being the center. Alternatively, the second laser beamL2 may be projected in a helical shape toward the center C.

Just as described, as shown in FIG. 6E and FIG. 10B, which will bedescribed below, in the solidified section (the weld section) G in whichthe melted section Y is solidified, a first columnar crystal regionformed of the columnar crystal structure G1 is formed in the directionfrom the peripheral edge S of the solidified section G toward the centerC of the solidified section G, and in the columnar crystal structure G1,the columnar crystal extends from the peripheral edge S of thesolidified section G. Furthermore, the equiaxed crystal region formed ofthe equiaxed crystal structure G2 is formed to surround the center C ofthe solidified section G from the first columnar crystal region (thecolumnar crystal structure G1). Moreover, the second columnar crystalregion formed of the columnar crystal structure G3 is formed from theequiaxed crystal region (the equiaxed crystal structure G2) to thecenter C of the solidified section G, and in the columnar crystalstructure G3, the columnar crystal extends toward the center C of thesolidified section G.

Accordingly, even when a crack that starts from the center C isgenerated in the columnar crystal structure of the second columnarcrystal region (the columnar crystal structure G3) during thesolidification, the extension of the crack can be reduced. It is becausethe growth of the columnar crystal is intermittent. In addition, evenwhen the crack further extends, this crack can be stopped by theequiaxed crystal structure G2 of the equiaxed crystal region. As aresult, the extension of the crack can be suppressed.

Here, the second laser beam may be used as described in the modificationof the first embodiment. In this case, the irradiation start regionneeds to be a region, a peripheral edge of which is located between theperipheral edge S of the melted section Y and the center C of the meltedsection Y and which includes the center C.

FIG. 7 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the second embodiment, in which FIG. 7A and FIG. 7B areviews of the change in the solidification of the melted section, inwhich FIG. 7C and FIG. 7D show that the workpieces are irradiated withthe second laser beam in this order, and in which FIG. 7E is a view ofthe weld section after the irradiation of the second laser beam.

In this modification, as shown in FIG. 7A to FIG. 7D, in the secondlaser beam irradiation process, the second laser beam L2 is projected inconjunction with the progression of the solidification immediatelybefore the equiaxed crystal is formed or at the same time as theequiaxed crystal is formed such that the equiaxed crystal does notremain and that the columnar crystal structure G1 grows from theperipheral edge S of the melted section Y to the center C of the meltedsection Y. As a result, the same weld section (the solidified section G)as that in the first embodiment can be obtained.

The solidification rate of the peripheral edge S of the melted section Ycan be brought closer to that of the center C of the melted section Y.Thus, as shown in FIG. 7E, the solidified section G in which the meltedsection Y is solidified is formed of the columnar crystal structure fromthe peripheral edge S of the solidified section G to the center of thesolidified section G, and in the columnar crystal structure, thecolumnar crystal extends in the direction from the peripheral edge S ofthe solidified section G toward the center C of the solidified sectionG. As a result, in the columnar crystal structure of the solidifiedsection G, even when the crack that starts from the center C isgenerated, the extension of the crack can be reduced. It is because thegrowth of each of the columnar crystals from the peripheral edge S ofthe solidified section G to the center of the solidified section G isintermittent.

Third Embodiment

FIG. 8 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of a thirdembodiment, in which FIG. 8A is a view of the solidified section afterthe irradiation of the first laser beam, in which FIG. 8B and FIG. 8Cshow that the workpieces are irradiated with the second laser beam inthis order, and in which FIG. 8D is a view of the solidified sectionafter the irradiation of the second laser beam.

The third embodiment differs from the first and second embodiments in apoint that the second laser beam is projected after the melted sectionis solidified. In this embodiment, after the solidified section G, inwhich the melted section Y is solidified, is formed (that is, see FIG.3B and FIG. 8A), the second laser beam L2 is projected such that thesecond laser beam L2 circles around the center C from the irradiationstart position that is deviated from the center C of the solidifiedsection G toward the center C. In this way, the irradiated part ismelted again, and thus a crack that is formed from the center C of thesolidified section G toward the peripheral edge S of the solidifiedsection G can be sealed.

More specifically, in the second laser beam irradiation process, thesecond laser beam L2 is projected onto the solidified section G andmelts the solidified section G again. Here, a position on a peripheraledge of a region that includes the center C of the solidified section Gand the vicinity thereof is set as the irradiation start positions suchthat the solidification rate until the region that includes the centerof the solidified section and the vicinity thereof and that is meltedagain by the irradiation of the second laser beam L2 is solidified islower than the solidification rate until the region that includes thecenter C of the melted section Y and the vicinity thereof is solidifiedafter the irradiation of the first laser beam L1 (the cooling rate inthe state shown in FIG. 3B). At this time, for the cooling rate, theintensity of the second laser beam is adjusted, heat absorbed by thesolidified section G is also adjusted. In addition, similar to the firstembodiment, the second laser beam L2 may be projected to draw theconcentric circles with the gradually reduced radii with the center Cbeing the center. Alternatively, the second laser beam L2 may beprojected in the helical shape toward the center C.

In this embodiment, as described above, the solidified section Gincludes: the equiaxed crystal region that is formed with the equiaxedcrystal structure G2 in the manner to include the center of thesolidified section G; and the columnar crystal region that is formed ofthe columnar crystal structure G1 from the peripheral edge S of thesolidified section G toward the equiaxed crystal region in a manner tosurround the equiaxed crystal region. Thus, as shown in FIG. 8B, thesecond laser beam is projected onto the equiaxed crystal region andmelts the equiaxed crystal region again (the melted section Y is formed)such that a part of the equiaxed crystal structure G2 of the equiaxedcrystal region surrounds the center of the solidified section G and thatthe remaining equiaxed crystal structure in the equiaxed crystal regionbecomes the columnar crystal structure G3.

Just as described, similar to the second embodiment, in the solidifiedsection (the weld section) G in which the melted section is solidified,the first columnar crystal region that is formed of the columnar crystalstructure G1 is formed in the direction from the peripheral edge S ofthe solidified section G toward the center C of the solidified sectionG, and in the columnar crystal structure G1, the columnar crystalextends from the peripheral edge S of the solidified section G.Furthermore, the equiaxed crystal region formed of the equiaxed crystalstructure G2 is formed to surround the center C of the solidifiedsection G from the first columnar crystal region (the columnar crystalstructure G1). Moreover, the second columnar crystal region formed ofthe columnar crystal structure G3 is formed from the equiaxed crystalregion (the equiaxed crystal structure G2) to the center C of thesolidified section G, and in the columnar crystal structure G3, thecolumnar crystal extends toward the center C of the solidified sectionG.

Accordingly, even when a crack that starts from the center C isgenerated in the columnar crystal structure of the second columnarcrystal region (the columnar crystal structure G3) during thesolidification, the extension of the crack can be reduced. It is becausethe growth of the columnar crystal is intermittent. In addition, evenwhen the crack further extends, this crack can be stopped by theequiaxed crystal structure G2 of the equiaxed crystal region. As aresult, the extension of the crack can be suppressed.

Similarly, also when a method for irradiating the laser beam as in themodification of the first embodiment is used to focus the second laserbeam from the irradiation start region that includes the center C of thesolidified section G and the periphery thereof (more specifically, theregion on an inner side of the equiaxed crystal region) to the center Cof the solidified section G, the crack that is formed from the center Cof the solidified section G toward the peripheral edge S of thesolidified section G can be sealed.

FIG. 9 includes schematic views for illustrating the second laser beamirradiation process according to the laser welding method of amodification of the third embodiment, in which FIG. 9A is a view of thesolidified section after the irradiation of the first laser beam, inwhich FIG. 9B and FIG. 9C show that the workpieces are irradiated withthe second laser beam in this order, and in which FIG. 9D is a view ofthe solidified section after the irradiation of the second laser beam.

In the above-described embodiment, the second laser beam L2 is projectedsuch that a part of the equiaxed crystal structure G2 remains. However,as shown in FIG. 9B and FIG. 9C, the second laser beam may be projectedsuch that the equiaxed crystal structure G2 of the equiaxed crystalregion becomes the columnar crystal structure G1.

In this way, in an obtained weld structure, the entire equiaxed crystalstructure becomes the columnar crystal structure in a manner to includethe center C of the solidified section (the weld section) G.Accordingly, the solidified section G is formed of the columnar crystalstructure from the peripheral edge S of the solidified section G to thecenter of the solidified section G, and in the columnar crystalstructure, the columnar crystal extends in the direction from theperipheral edge S of the solidified section G toward the center C of thesolidified section G. As a result, in the columnar crystal structure G3of the solidified section G, even when the crack that starts from thecenter C is generated, the extension of the crack can be reduced. It isbecause the growth of each of the columnar crystals from the peripheraledge S of the solidified section G to the center of the solidifiedsection G is intermittent.

Noted that, in any of the embodiments, the laser beam is projected ontothe melted section or the center of the solidified section for a longtime in the weld structure that includes the weld section (thesolidified section G) that is welded by the first and second laserbeams. Accordingly, in any of the embodiments, as exemplified in FIG.9C, for example, a surface of the solidified section G on a side that isirradiated with the laser beam is formed of a primary recessed surfaceF1 that is recessed from the peripheral edge S of the solidified sectionG toward the center of the weld section and a secondary recessed surfaceF2 that is further recessed from the primary recessed surface F1 in thevicinity of the center of the solidified section G. In addition, a backsurface of the solidified section G that corresponds to the secondaryrecessed surface F2 is formed of a recessed surface F3 that is shallowerthan the secondary recessed surface F2. In addition, as shown in theother embodiments, the back surface of the solidified section G thatcorresponds to the secondary recessed surface F2 may not be recessed(see FIG. 7E, for example).

As a result, the secondary recessed surface F2 is formed on the surfaceon the side that is irradiated with the laser beam. Thus, the secondaryrecessed surface F2 can easily be repaired by a sealer or the like afterwelding. In addition, the back surface of the weld section thatcorresponds to the secondary recessed surface F2 is either not recessedor is formed of the recessed surface F3 that is shallower than thesecondary recessed surface F2. Thus, water or the like is less likely tobe collected therein, and a chance of corrosion of the part can bereduced.

EXAMPLES

Examples according to the invention will be described below.

First Example (Corresponding to FIG. 4)

Plate materials (workpieces) that are made of 6000 series aluminum alloyand respectively have thicknesses of 1.2 mm and 1.0 mm were prepared.The first laser beam was projected thereon to have a weld radius of 8mm. Furthermore, by using the method shown in the first embodiment(shown in FIG. 4), a scanning trajectory of the second laser beam wasgradually focused from the outer periphery of the melted section to theinner side, and the melted section was slowly cooled for solidificationto form the weld section in each of these materials. The first laserbeam had output of 2.5 to 4.0 kw, and the second laser beam had outputin a range of 0.5 to 1.5 kw. Similarly, the plate materials as theworkpieces were bent in an L shape and welded as shown in FIG. 14.

Second Example (see FIG. 6)

The plate materials as the workpieces were welded in the same manner asin the first example. The second example differed from the first examplein a point that, in the method described in the second embodiment (seeFIG. 6), the irradiation of the laser beam remained stopped in a periodafter the projection of the first laser beam and before thesolidification of the center of the melted section until the formationof the equiaxed crystal was started, then the laser beam was projectedagain after the formation of the equiaxed crystal, and the region on theinner side of the equiaxed crystal was slowly cooled to solidify themelted section (to form the weld section).

Comparative Example

The plate materials as the workpieces were welded in the same manner asin the first example. The comparative example differed from the firstexample in a point that the second laser beam was not projected.

<Observation of Structures>

The weld sections according to the first and second examples and thecomparative example were subject to electropolishing and then observedwith a microscope. FIG. 10A is an image of the structure of the weldsection according to the first example, FIG. 10B is an image of thestructure of the weld section according to the second example, and FIG.10C is an image of the structure of the weld section according to thecomparative example.

As shown in FIG. 10A, it can be understood that the weld section of thefirst example was formed of the columnar crystal structure from theperipheral edge of the weld section to the center of the weld section,and in the columnar crystal structure, the columnar crystal extended inthe direction from the peripheral edge of the weld section toward thecenter of the weld section.

As shown in FIG. 10B, in the weld section of the second example, thefirst columnar crystal region that was formed of the columnar crystalstructure was formed, and in the columnar crystal structure, thecolumnar crystal extended from the peripheral edge of the weld sectionin the direction from the peripheral edge of the weld section toward thecenter of the weld section. In addition, the equiaxed crystal regionthat was formed of the equiaxed crystal structure was formed from thefirst columnar crystal region, and the equiaxed crystal structure beingformed to surround the center of the weld section. Furthermore, thesecond columnar crystal region that was formed of the columnar crystalstructure was formed from the equiaxed crystal region to the center ofthe weld section, and in the columnar crystal structure, the columnarcrystal extended toward the center of the weld section.

As shown in FIG. 10C, in the weld section of the comparative example,the equiaxed crystal structure was formed to include the center of theweld section, and the columnar crystal structure was formed from theperipheral edge of the weld section toward the equiaxed crystal regionin a manner to surround the equiaxed crystal region.

<Observation of Cross Sections>

The cross sections of the weld sections according to the first exampleand the comparative example were observed with the microscope. FIG. 11Ais an image of a cross section of the weld section according to thefirst example, and FIG. 11B is an image of the cross section of the weldsection according to the comparative example.

As shown in FIG. 11A, in the weld section of the first example, thesurface of the weld section on the side that was irradiated with thelaser beam was formed with the primary recessed surface F1 that wasrecessed from the peripheral edge of the weld section toward the centerof the weld section and the secondary recessed surface F2 that wasfurther recessed from the primary recessed surface F1 in the vicinity ofthe center of the weld section. In addition, the back surface of thesolidified section G that corresponded to the secondary recessed surfaceF2 was formed with the recessed surface F3 that was shallower than thesecondary recessed surface F2.

As shown in FIG. 11B, in the weld section of the comparative example,the surface of the weld section on the side that is irradiated with thelaser beam was formed with the primary recessed surface F1 that wasrecessed from the peripheral edge of the weld section toward the centerof the weld section. However, the secondary recessed surface F2 that wasfurther recessed from the primary recessed surface F1 in the vicinity ofthe center of the weld section as described in the first example was notformed. Differing from the back surface of the solidified section G inthe first example, the back surface of the solidified section G thatcorresponded to the secondary recessed surface F2 was formed with therecessed surface F3 that was deeper than the secondary recessed surfaceF2.

<Observation of Fractures>

Three samples were used to measure maximum lengths of the fracture andtotal lengths of the fracture with respect to the weld sectionsaccording to the first and second examples and the comparative example.FIG. 12A shows the maximum lengths of the fracture in the weld sectionsaccording to the first example, the second example, and the comparativeexample, and FIG. 12B shows the total lengths of the fractures in theweld sections according to the first example, the second example, andthe comparative example.

As shown in FIG. 12A and FIG. 12B, the maximum length of the fractureand the total length of the fracture were short in an order of thesecond example, the first example, and the comparative example. It isconsidered that, in the case of the first example, in comparison withthe comparative example, the columnar crystal structure was formed fromthe peripheral edge of the solidified section to the center of thesolidified section, the growth of each of the columnar crystals in thecolumnar crystal structure was intermittent, and thus the extension ofthe crack could be reduced. It is also considered that, in the case ofthe second example, in comparison with the first example and thecomparative example, even when the crack that started from the centerwas generated in the columnar crystal structure of the second columnarcrystal region during the solidification, this crack could be stopped bythe equiaxed crystal structure of the equiaxed crystal region.

<Observation of Fracture Morphology>

With respect to the first example and the comparative example, states ofthe surface at a time when a shearing load acted on the weld sectionswere observed. FIG. 13A is an image of the surface of the weld sectionaccording to the second example when a shearing load acted on the weldsection, and FIG. 13B is an image of the surface of the fracture in theweld section according to the comparative example when the shearing loadacted on the weld section.

For the first example and the comparative example, states of the surfaceat a time when a tensile load acted on the weld section were observed.FIG. 14A is an image of the surface of the weld section according to thesecond example when the tensile load acted on the weld section, and FIG.14B is an image of the surface of the fracture in the weld sectionaccording to the comparative example when the tensile load acted on theweld section.

As shown in FIG. 13A and FIG. 13B, in the case of the second example,the fracture was generated at the peripheral edge of the weld section (aboundary section). However, in the case of the comparative example, thefracture was generated at the center of the weld section. In otherwords, in the second example, the fracture was not generated from thecrack that was generated by welding.

As shown in FIG. 14B, in the case of the second example, the fracturewas not generated. However, in the case of the comparative example, thefracture was generated at the center of the weld section.

The description has been made so far by using the embodiments of theinvention. Specific configurations thereof are not limited to theseembodiments and examples, and any design changes that are made withinthe scope of the gist of the invention are included in the invention.

What is claimed is:
 1. A laser welding method comprising: forming amelted section in which plural metallic workpieces within a weld regionare melted by projecting a first laser beam onto said weld region withparts of the workpieces being set as the weld region when the workpiecesare welded; and projecting a second laser beam while the melted sectionis being solidified or after a solidified section in which the meltedsection is solidified is formed such that the second laser beam circlesaround a center of the melted section or the solidified section from anirradiation start position deviated from said center toward said centeror such that the second laser beam is focused to the center of themelted section or the solidified section from an irradiation startregion that includes the center and a periphery thereof, wherein themelted section including the center is progressed to solidify, or thesolidified section is re-melted and a re-melted section is progressed tosolidify while projecting the second laser beam; and a solidificationrate of at least the center of the melted section or the re-meltedsection is lowered by the projection of the second laser beam.
 2. Thelaser welding method according to claim 1, wherein the second laser beamis projected until the melted section is solidified, and in irradiationof the second laser beam, the second laser beam is projected onto themelted section along with progression of solidification in a directionfrom a peripheral edge of the melted section toward the center of themelted section, so that the progression of the solidification in thedirection from the peripheral edge of the melted section toward thecenter of the melted section is delayed.
 3. The laser welding methodaccording to claim 2, wherein in the irradiation of the second laserbeam, the irradiation start position is located on the peripheral edgeof the melted section, or the irradiation start region is a region thatis surrounded by the peripheral edge of the melted section.
 4. The laserwelding method according to claim 2, wherein in the irradiation of thesecond laser beam, the irradiation start position is located between theperipheral edge of the melted section and the center of the meltedsection, or the irradiation start region is a region, a peripheral edgeof which is located between the peripheral edge of the melted sectionand the center of the melted section and which includes the center. 5.The laser welding method according to claim 3, wherein in theirradiation of the second laser beam, the second laser beam is projectedsuch that a columnar crystal structure grows from the peripheral edge ofthe melted section to the center of the melted section in conjunctionwith the progression of the solidification.
 6. The laser welding methodaccording to claim 4, wherein a second laser beam irradiation is startedafter a columnar crystal structure grows from the peripheral edge of themelted section toward the center of the melted section so as to surroundthe center of the melted section in conjunction with the progression ofthe solidification and then growth of an equiaxed crystal structure isstarted after growth of said columnar crystal structure is completed ina period after irradiation of the first laser beam and before theirradiation of the second laser beam, and in the irradiation of thesecond laser beam, the second laser beam is projected such that theequiaxed crystal structure remains in a manner to surround the center ofthe melted section and that the columnar crystal structure grows fromsaid equiaxed crystal structure to the center of the melted section. 7.The laser welding method according to claim 1, wherein in theirradiation of the second laser beam, the second laser beam is projectedonto the solidified section in which the melted section is solidified,so as to melt the solidified section again, and the second laser beam isprojected such that a cooling rate, in which the region that includesthe center of the solidified section and a vicinity thereof and that ismelted again by the irradiation of the second laser beam is solidified,is slower than a cooling rate, in which the region that includes thecenter of the melted section and the vicinity thereof is solidifiedafter the irradiation of the first laser beam, over a position on aperipheral edge of the region that includes the center of the solidifiedsection and the vicinity thereof being set as the irradiation startposition, or over a region that is surrounded by the peripheral edge ofthe region that includes the center of the solidified section and thevicinity thereof being set as the irradiation start region.
 8. The laserwelding method according to claim 7, wherein the solidified sectionincludes: an equiaxed crystal region that is formed with an equiaxedcrystal structure in a manner to include the center of the solidifiedsection; and a columnar crystal region that is formed with a columnarcrystal structure in a manner to surround the equiaxed crystal regionfrom the peripheral edge of the solidified section toward the equiaxedcrystal region, and the second laser beam is projected such that theequiaxed crystal structure of the equiaxed crystal region becomes thecolumnar crystal structure.
 9. The laser welding method according toclaim 7, wherein the solidified section includes: an equiaxed crystalregion that is formed with an equiaxed crystal structure in a manner toinclude the center of the solidified section; and a columnar crystalregion that is formed with a columnar crystal structure in a manner tosurround the equiaxed crystal region from the peripheral edge of thesolidified section toward the equiaxed crystal region, and the secondlaser beam is projected onto the equiaxed crystal region such that apart of the equiaxed crystal structure of the equiaxed crystal regionsurrounds the center of the solidified section and such that theremaining equiaxed crystal structure of the equiaxed crystal regionbecomes the columnar crystal structure.
 10. A laser welding methodcomprising: forming a melted section in which plural metallic workpieceswithin a weld region are melted by projecting a first laser beam ontosaid weld region with parts of the workpieces being set as the weldregion when the workpieces are welded; and projecting a second laserbeam while the melted section is being solidified or after a solidifiedsection in which the melted section is solidified is formed such thatthe second laser beam circles around a center of the melted section orthe solidified section from an irradiation start position deviated fromsaid center toward said center or such that the second laser beam isfocused to the center of the melted section or the solidified sectionfrom an irradiation start region that includes the center and aperiphery thereof, wherein in the irradiation of the second laser beam,the irradiation start position is located between a peripheral edge ofthe melted section and the center of the melted section, or theirradiation start region is a region, a peripheral edge of which islocated between the peripheral edge of the melted section and the centerof the melted section and which includes the center.
 11. A laser weldingmethod comprising: forming a melted section in which plural metallicworkpieces within a weld region are melted by projecting a first laserbeam onto said weld region with parts of the workpieces being set as theweld region when the workpieces are welded; and projecting a secondlaser beam while the melted section is being solidified or after asolidified section in which the melted section is solidified is formedsuch that the second laser beam circles around a center of the meltedsection or the solidified section from an irradiation start positiondeviated from said center toward said center or such that the secondlaser beam is focused to the center of the melted section or thesolidified section from an irradiation start region that includes thecenter and a periphery thereof, wherein in the irradiation of the secondlaser beam, the second laser beam is projected such that a columnarcrystal structure grows from a peripheral edge of the melted section tothe center of the melted section in conjunction with progression of thesolidification.